To be able to manufacture photovoltaic cells made of silicon, a phosphorus doping of the unfinished cells is first necessary. In the first step, the cell is wetted using phosphoric acid, and the wetted cells are placed in a high-temperature oven at ca. 800° to 900° C., where the phosphorus diffuses from the dried acid into the silicon substrate. The coating is designed to be very even to achieve equal distribution in the diffusion process, and it is also designed to be very economical since the excess phosphoric acid melts onto the cell as “phosphorus glass,” and it can only be removed using hydrofluoric acid, which is accomplished with difficulty.
Phosphoric acid is usually applied to the silicon substrates in such a way that the phosphoric acid is atomized by a high-frequency ultrasound device, and the phosphoric acid mist is deposited onto the silicon substrates. The phosphoric acid mist is conveyed from the process chamber into a dropping shaft, which is relatively wide and is arranged at a relatively large distance above the silicon substrates, i.e., cells, that are conveyed past it. One disadvantage in this known device lies in the fact that it offers no guarantee that the mist will be homogenized, because even light air currents are sufficient to “blow” the mist. In addition, the configuration of the process chamber leads to damaging condensate drops falling onto the silicon cells, which works against achieving a homogeneous wetting or coating result. The attempt has been made to catch condensate drops of this type using a drip pan beneath the dropping shaft, which however has the effect of preventing the mist even more from being evenly spread.